The opendir() function opens a directory stream corresponding to the
directory name, and returns a pointer to the directory stream. The
stream is positioned at the first entry in the directory.
The fdopendir() function is like opendir(), but returns a directory
stream for the directory referred to by the open file descriptor fd.
After a successful call to fdopendir(), fd is used internally by the
implementation, and should not otherwise be used by the application.

EACCES Permission denied.
EBADF fd is not a valid file descriptor opened for reading.
EMFILE The per-process limit on the number of open file descriptors
has been reached.
ENFILE The system-wide limit on the total number of open files has
been reached.
ENOENT Directory does not exist, or name is an empty string.
ENOMEM Insufficient memory to complete the operation.
ENOTDIRname is not a directory.

Filename entries can be read from a directory stream using
readdir(3).
The underlying file descriptor of the directory stream can be
obtained using dirfd(3).
The opendir() function sets the close-on-exec flag for the file
descriptor underlying the DIR *. The fdopendir() function leaves the
setting of the close-on-exec flag unchanged for the file descriptor,
fd. POSIX.1-200x leaves it unspecified whether a successful call to
fdopendir() will set the close-on-exec flag for the file descriptor,
fd.

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 OPENDIR(3)